This gets into my comment about all of the connections end to end. HTTPS encrypts the traffic between your web browser and the web server you're communicating with to prevent a 3rd party in the middle from viewing your data. It's not 100% guaranteed. Sometimes there are vulnerabilities that break HTTPS. Sometimes you're on a hostile network (even a corporate network at work!) that intercepts and inspects the data within your HTTPS connections. This would make for a nice info graphic showing all of the pieces involved with an email between client and sender. I have the start of one here (How to encrypt and send secure email) from a few years ago that could go into more detail.

An email client can be "secure" if it's reading email over IMAP+SSL or POP3+SSL. It also matters if you care about the copy of email being created on your device getting stored encrypted or not. Some of the newer encrypted email providers have apps that should be more secure than the standard email client such as Microsoft Outlook.

If one uses "https everywhere" will an non encrypted e mail provider/client be secure.

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It depends which part of the communication that you're trying to secure. Most web based email these days is already full session HTTPS. HTTPS everywhere does nothing to help with the email stored on the providers servers, your device(s), the senders device(s), the senders providers, or the connections between all of the above not already mentioned.